The effect of tumor-bearing on host tumor immunity will be studied in vivo and in vitro. The role of various "blocking factors" in the induction and reversal of tumor-mediated suppression of immune reactivity will be examined using the chromium-51 release assay for cytotoxicity. Experiments are proposed to determine the nature and basic characteristics of the factors responsible for the impairment of lymphocyte reactivity in tumor-bearing animals. In addition, the relationship of impaired lymphocyte reactivity, lymphocyte sequestration, and tumor burden will be studied. BIBLIOGRAPHIC REFERENCES: Laux, D. C. and H. M. Robinson. Cell-mediated cytotoxin reactivity to syngeneic tumors in normal and tumor-bearing hamsters. Proc. Amer. Assoc. Cancer Res. 17:62, 1976. Disciullo, S. O. and D. C. Laux. Resistance of PARA-7 tumor cells to the effects of antibody and complement. Abst. Amer. Assoc. Microbiol. 1976. Pg. 68.